1. Field of the Invention
The present invention relates to a biaxially oriented thermoplastic resin film, and more particularly to a biaxially oriented thermoplastic resin film suitable for the base film of a magnetic recording media such as a video tape.
2. Description of the Prior Art
The surface of a biaxially oriented thermoplastic resin film used for a magnetic recording media such as a video tape has been required to be flatter in order to obtain a higher output property of the tape. As a biaxially oriented thermoplastic resin film used for a magnetic recording media, a film wherein spherical silica particles are contained in polyester is known and described in, for example, JP-A-SHO 59-171623.
In such a conventional biaxially oriented thermoplastic resin film, however, there is a problem that lengthwise wrinkles and/or telescoping are likely to occur when the film is produced, slitted or processed into a video tape, when the surface of the film is formed to be flatter in order to satisfy the above requirement. On the other hand, when the surface of the film is formed roughly, the signal/noise ratio (S/N) when the film is processed into a video tape is not good and it is difficult to obtain a sufficiently high-quality image when the original video tape formed from the film is dubbed.
As a film solving the above problem, a biaxially oriented thermoplastic resin film, wherein protrusions with a relatively uniform height are formed on at least one surface of the film at a high density in order to achieve good handling ability and running ability and in order to suppress the deterioration of the image quality when the tape is dubbed, is known and described in, for example, JP-A-HEI 2-77431.
Further, a biaxially oriented thermoplastic resin film, wherein protrusions are formed on the film surface by using at least two kinds of particles having mean diameters different from each other in order to achieve the above properties as well as improve the scratch resistance, is known and described in, for example, JP-A-HEI 3-150127.
However, even in such conventional films, it is difficult to prevent a defect of roll formation in the film winding process and to increase the yield for film production or film processing thereby reducing the production or processing cost.